The Gradients in the 47 Tuc Red Giant Branch Bump and Horizontal Branch are Consistent With a Centrally-Concentrated, Helium-Enriched Second Stellar Generation

The Astrophysical Journal (Impact Factor: 5.99). 02/2011; 736(2). DOI: 10.1088/0004-637X/736/2/94
Source: arXiv


We combine ground and space-based photometry of the Galactic globular cluster
47 Tuc to measure four independent lines of evidence for a helium gradient in
the cluster, whereby stars in the cluster outskirts would have a lower initial
helium abundance than stars in and near the cluster core. First and second, we
show that the red giant branch bump (RGBB) stars exhibit gradients in their
number counts and brightness. With increased separation from the cluster
center, they become more numerous relative to the other red giant (RG) stars.
They also become fainter. For our third and fourth lines of evidence, we show
that the horizontal branch (HB) of the cluster becomes both fainter and redder
for sightlines farther from the cluster center. These four results are
respectively detected at the 2.3$\sigma$, 3.6$\sigma$, 7.7$\sigma$ and
4.1$\sigma$ levels. Each of these independent lines of evidence is found to be
significant in the cluster-outskirts; closer in, the data are more compatible
with uniform mixing. Our radial profile is qualitatively consistent with but
quantitatively tighter than previous results based on CN absorption. These
observations are qualitatively consistent with a scenario wherein a second
generation of stars with modestly enhanced helium and CNO abundance formed deep
within the gravitational potential of a cluster of previous generation stars
having more canonical abundances.

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Available from: Marc H. Pinsonneault, Feb 19, 2015
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    • "Bragaglia et al. (2010a) found that the luminosity of the RGB bump, which should increase with He abundance, is fainter in first generation than second generation in 14 globular clusters. Nataf et al. (2011) found that the RGB bump is brighter at the centre of 47 Tuc, where second generation stars should dominate . The colours of second generation RGB stars are bluer than those of first generation ones, in agreement with expectations for a higher He content (e.g., Bragaglia et al. 2010a; Sbordone et al. 2011). "
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